Watch the The Break online guitar lesson by Karan Andrea from Guitar Player Wanted: Vocals A Plus
OK, so you probably do just fine singing low notes - things in your natural speaking range - but the higher you go up the scale, the harder it is to hit the notes, and it might hurt a bit or feel strained. Maybe you've noticed that a couple of notes are almost impossible to hit, or aren't reliably there for you.
So what about those pesky notes in the middle that are making you nuts? Well, those notes collectively are called "the break." In classical training it is called the passaggio or passage voice. This is the shift between your chest and head voice. It never completely goes away, but as you learn to coordinate the singing muscles, and continue to fine-tune them, you will be able to sing through that break and no one will know but you.
Actually, this 'break' is the primary reason why so many people spend so much money and time with voice lessons and instructional materials. Learning to sing is all about coordinating the muscles involved. As you sing and as you practice, you are teaching your muscles to work together effectively. You are creating muscle memory. Just like learning to play guitar, learning to sing is all about muscle coordination - there should never be any mystery to it.